The present invention relates to a microwave (hereinafter referred to as “MW”) sensor system that is an active sensor using magnetic waves of a lower frequency than that of visible light rays.
At present, as one security device, an MW sensor system is known which, when MWs are transmitted toward a detection area and a target (e.g., rain, snow, human bodies) serving as the detection target is present in the detection area, receives waves (MWs modulated by the Doppler effect) reflected from the target to detect the target.
This MW sensor system is disposed with a transmitter and a receiver that transmit and receive MWs. When a target is present in the detection area, the MWs transmitted from the transmitter towards the detection area are received by the receiver after the frequencies of the waves reflected from the target are modulated by the Doppler effect. In other words, in this case, because the MWs received by the receiver are modulated with respect to the frequency of the MWs transmitted from the transmitter, a change arises in the output signal waveform of the MW sensor system, whereby the target detection signal is transmitted from the MW sensor system (e.g., see Japanese Patent Application Laid-Open Publication No. Hei6-347564).
Among MW sensor systems, there is a system provided with two sensors that transmit MWs toward a detection area and receive MWs reflected from this detection area.
In this MW sensor system provided with two sensors, the phases of the MWs transmitted from the sensors are respectively different (dual-frequency system). Due to this phase difference and a preset frequency difference (referred to below as “Δf”), a target in a preset detection area is detected. Because this system uses sensors in which the phases of the MWs are respectively different, it can more reliably measure the distance to the target in comparison to an MW sensor system disposed with one sensor.
However, if there is an object between the sensors and the target, the dual-frequency MW sensor system is affected by that object. For example, in a case where rain is present in the air between the sensors and the target as in a rainfall, that rain is detected and appears as noise in the IF (Intermediate Frequency) output signals.
That is, the distance from the sensors to the detected rain ends up being measured rather than the distance from the sensors to the target. For this reason, the rain is outputted as noise and the phase difference of the two IF output signals of the rain noise becomes small, which is a reason why rain noise lowers detection accuracy, as shown in FIG. 6.
FIG. 6 is a graph showing the relation between the phase difference (represented below as “Δø”) of target detection signals, in a case where Δf equals 5 MHz, and the distance from the sensors to the detection area. The single-dot line in FIG. 6 represents the phase difference in the case of clear weather (i.e., the phase difference that the system originally has in a state not affected by weather), the solid line represents the phase difference in the case of rainy weather, and the dotted lines represent a case where Δø is n and the distance from the sensors to the detection area (represented below as “R”) is 15 m. Also, as conditions for the measurement, the maximum of R is set to 15 m, the target signal is set to 200 mV (represented below as “S”) and the rain noise is set to 140 mV (represented below as “N”), so that S/N=3 dB.
As shown in FIG. 6, when R is 15 m, a detection error of 30% arises in the case of rainy weather compared to the case of clear weather. For this reason, it has not been possible to increase the distance from the sensors to the detection area.
Thus, in order to solve this problem, it is an object of the present invention to provide a microwave sensor system that reduces detection error during rainy weather and with which the distance from the sensors to the detection area is optionally set.